iwlwifi: cleanup HT40 extension channels setup

The patch cleans up the HT40 extension channels setup for EEPROM
band 6 and 7 to make it more readable.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>